Tags: debugging



Wednesday, August 5th, 2020

In a Land Before Dev Tools | Amber’s Website

A great little history lesson from Amber—ah, Firebug!

Wednesday, June 17th, 2020

Where did the focus go? | Amber’s Website

Amber documents a very handy bit of DOM scripting when it comes to debugging focus management: document.activeElement.

Monday, January 14th, 2019

djinn and juice : The Best Debugging Story I’ve Ever Heard

Cassie and I were swapping debugging stories. I shared the case of the 500 mile email with her. She shared this with me.

Sunday, October 28th, 2018

Phil Nash and Jeremy Keith Save the Safari Video Playback Day

I love this example of paying it forward:

Friday, October 26th, 2018

Service workers and videos in Safari

Alright, so I’ve already talked about some gotchas when debugging service worker issues. But what if you don’t even realise the problem has anything to do with your service worker?

This is not a hypothetical situation. I encountered this very thing myself. Gather ‘round the campfire, children…

One of the latest case studies on the Clearleft site is a nice write-up by Luke of designing a mobile app for Virgin Holidays. The case study includes a lovely video that demonstrates the log-in flow. I implemented that using a video element (with a poster image). Nice and straightforward. Super easy. All good.

But I hadn’t done my due diligence in browser testing (I guess I didn’t even think of it in this case). Hana informed me that the video wasn’t working at all in Safari. The poster image appeared just fine, but when you clicked on it, the video didn’t load.

I ducked, ducked, and went, uncovering what appeared to be the root of the problem. It seems that Safari is fussy about having servers support something called “byte-range requests”.

I had put the video in question on an Amazon S3 server. I came to the conclusion that S3 mustn’t support these kinds of headers correctly, or something.

Now I had a diagnosis. The next step was figuring out a solution. I thought I might have to move the video off of S3 and onto a server that I could configure a bit more.

Luckily, I never got ‘round to even starting that process. That’s good. Because it turns out that my diagnosis was completely wrong.

I came across a recent post by Phil Nash called Service workers: beware Safari’s range request. The title immediately grabbed my attention. Safari: yes! Video: yes! But service workers …wait a minute!

There’s a section in Phil’s post entitled “Diagnosing the problem”, in which he says:

I first thought it could have something to do with the CDN I’m using. There were some false positives regarding streaming video through a CDN that resulted in some extra research that was ultimately fruitless.

That described my situation exactly. Except Phil went further and nailed down the real cause of the problem:

Nginx was serving correct responses to Range requests. So was the CDN. The only other problem? The service worker. And this broke the video in Safari.

Doh! I hadn’t even thought about service workers!

Phil came up with a solution, and he has kindly shared his code.

I decided to go for a dumber solution:

if ( request.url.match(/\.(mp4)$/) ) {

That tells the service worker to just step out of the way when it comes to video requests. Now the video plays just fine in Safari. It’s a bit of a shame, because I’m kind of penalising all browsers for Safari’s bug, but the Clearleft site isn’t using much video at all, and in any case, it might be good not to fill up the cache with large video files.

But what’s more important than any particular solution is correctly identifying the problem. I’m quite sure I never would’ve been able to fix this issue if Phil hadn’t gone to the trouble of sharing his experience. I’m very, very grateful that he did.

That’s the bigger lesson here: if you solve a problem—even if you think it’s hardly worth mentioning—please, please share your solution. It could make all the difference for someone out there.

Service workers and browser extensions

I quite enjoy a good bug hunt. Just yesterday, myself and Cassie were doing some bugfixing together. As always, the first step was to try to reproduce the problem and then isolate it. Which reminds me…

There’ve been a few occasions when I’ve been trying to debug service worker issues. The problem is rarely in reproducing the issue—it’s isolating the cause that can be frustrating. I try changing a bit of code here, and a bit of code there, in an attempt to zero in on the problem, butwith no luck. Before long, I’m tearing my hair out staring at code that appears to have nothing wrong with it.

And that’s when I remember: browser extensions.

I’m currently using Firefox as my browser, and I have extensions installed to stop tracking and surveillance (these technologies are usually referred to as “ad blockers”, but that’s a bit of a misnomer—the issue isn’t with the ads; it’s with the invasive tracking).

If you think about how a service worker does its magic, it’s as if it’s sitting in the browser, waiting to intercept any requests to a particular domain. It’s like the service worker is the first port of call for any requests the browser makes. But then you add a browser extension. The browser extension is also waiting to intercept certain network requests. Now the extension is the first port of call, and the service worker is relegated to be next in line.

This, apparently, can cause issues (presumably depending on how the browser extension has been coded). In some situations, network requests that should work just fine start to fail, executing the catch clauses of fetch statements in your service worker.

So if you’ve been trying to debug a service worker issue, and you can’t seem to figure out what the problem might be, it’s not necessarily an issue with your code, or even an issue with the browser.

From now on when I’m troubleshooting service worker quirks, I’m going to introduce a step zero, before I even start reproducing or isolating the bug. I’m going to ask myself, “Are there any browser extensions installed?”

I realise that sounds as basic as asking “Are you sure the computer is switched on?” but there’s nothing wrong with having a checklist of basic questions to ask before moving on to the more complicated task of debugging.

I’m going to make a checklist. Then I’m going to use it …every time.

Wednesday, August 15th, 2018

Google AMP - A 70% drop in our conversion rate. - Rockstar Coders

Google hijacking and hosting your AMP pages (in order to pre-render them) is pretty terrible for user experience and security:

I’m trying to establish my company as a legitimate business that can be trusted by a stranger to build software for them. Having google.com reeks of a phishing scam or fly by night operation that couldn’t afford their own domain.

Tuesday, August 7th, 2018

Console methods

Whenever I create a fetch event inside a service worker, my code roughly follows the same pattern. There’s a then clause which gets executed if the fetch is successful, and a catch clause in case anything goes wrong:

fetch( request)
.then( fetchResponse => {
    // Yay! It worked.
.catch( fetchError => {
    // Boo! It failed.

In my book—Going Offline—I’m at pains to point out that those arguments being passed into each clause are yours to name. In this example I’ve called them fetchResponse and fetchError but you can call them anything you want.

I always do something with the fetchResponse inside the then clause—either I want to return the response or put it in a cache.

But I rarely do anything with fetchError. Because of that, I’ve sometimes made the mistake of leaving it out completely:

fetch( request)
.then( fetchResponse => {
    // Yay! It worked.
.catch( () => {
    // Boo! It failed.

Don’t do that. I think there’s some talk of making the error argument optional, but for now, some browsers will get upset if it’s not there.

So always include that argument, whether you call it fetchError or anything else. And seeing as it’s an error, this might be a legitimate case for outputing it to the browser’s console, even in production code.

And yes, you can output to the console from a service worker. Even though a service worker can’t access anything relating to the document object, you can still make use of window.console, known to its friends as console for short.

My muscle memory when it comes to sending something to the console is to use console.log:

fetch( request)
.then( fetchResponse => {
    return fetchResponse;
.catch( fetchError => {

But in this case, the console.error method is more appropriate:

fetch( request)
.then( fetchResponse => {
    return fetchResponse;
.catch( fetchError => {

Now when there’s a connectivity problem, anyone with a console window open will see the error displayed bold and red.

If that seems a bit strident to you, there’s always console.warn which will still make the output stand out, but without being quite so alarmist:

fetch( request)
.then( fetchResponse => {
    return fetchResponse;
.catch( fetchError => {

That said, in this case, console.error feels like the right choice. After all, it is technically an error.

Monday, April 30th, 2018

Apart From Code

A good developer…

  • debugs
  • follows the KISS principle (and respects YAGNI)
  • knows how to research
  • works well with others
  • finds good developer tools
  • tests code

Friday, July 21st, 2017

Patterns Day 2017: Rachel Andrew on Vimeo

Rachel’s fantastic talk from Patterns Day. There’s a lot of love for Fractal specifically, but there are also some great points about keeping a pattern library in sync with a live site, and treating individual components as reduced test-cases.

Sunday, April 10th, 2016

A debugging thought process

Remy walks us through his performance debugging routine …and now Una must write him a song.

Friday, January 22nd, 2016

The Art of Debugging

I was just helping out with some debugging at work and it reminded me of this great talk/post by Remy:

  1. Replicate: see the bug
  2. Isolate: understand the bug
  3. Eliminate: fix the bug

Monday, November 16th, 2015

Online regex tester and debugger: JavaScript, Python, PHP, and PCRE

Regular expressions are my kryptonite. I’m rubbish at them and I can never keep the vocabulary in me head.

Mark recommended this tool so I’m going to give it a go the next time I have to resort to regex.

Friday, January 2nd, 2015

Taking Chrome DevTools outside of the browser. — Kenneth Auchenberg

Kenneth has isolated Chrome’s dev tools into its own app. This is a big step towards this goal:

Why are DevTools still bundled with the browsers? What if clicking “inspect element” simply started an external DevTools app?

With DevTools separated from one specific browser, a natural next step would be making the DevTools app work with other browsers.

Wednesday, March 7th, 2012

Adobe Shadow | preview mobile web - Adobe Labs

Adobe have launched their version of Weinre, the tool that allows you to refresh and debug iOS and Android browser views from your desktop computer.

Friday, October 17th, 2008

paulhammond.org: Conditional classnames

Here's a handy little trick from Paul: use conditional comments to add a class to your BODY element, allowing you to target IE without a separate stylesheet.